Grain Acceleration by MHD Turbulence: Gyroresonance Mechanism

We discuss a new mechanism of dust acceleration that acts in a turbulent magnetized medium. The magnetohydrodynamic (MHD) turbulence includes both fluid motions and magnetic fluctuations. We show that while the fluid motions bring about grain motions through the drag, the electromagnetic fluctuations, can accelerate grains through resonant interactions. In this paper we calculate grain acceleration by gyroresonance in Cold Neutral Medium. We consider both incompressible and compressible MHD modes. We show that fast modes dominate grain acceleration. For the parameters chosen, fast modes render supersonic velocities to grains that may shatter grains and enable efficient absorption of heavy elements. Since the grains are preferentially accelerated with large pitch angles, the supersonic grains get aligned with long axes perpendicular to the magnetic field.